1. Technical Field
This invention relates to method and apparatus for installing underground duct banks. In one aspect, the invention relates to method and apparatus for installing duct banks utilized for carrying and protecting underground electrical wiring and cabling systems in construction projects.
2. Background
The transmission of electrical power, electrical control signals, and telephone communications is carried out through wires and cables installed inside conduits called ducts from one point of a construction project to another, including to and through buildings under construction. These ducts are assembled in banks called conduit banks or duct banks. The distances involved can be from a few hundred feet to a few miles, and the conventional duct bank is assembled inside an underground trench.
For long distances, precast concrete manholes are installed at intervals of about 500 feet or more, thereby to facilitate pulling the cables through the inside of the ducts (conduits). The precast concrete manholes generally are installed prior to excavating the trench. The precast concrete manholes are used to pull the cables through the ducts (conduits) and for connecting the pulled cables to the next lengths of cables pulled through the next length of duct bank. Cable pulling through the ducts (conduits) is always done after the duct bank installation is complete. These ducts (conduits) typically are made of plastic material which give them some flexibility.
Conventional duct banks are assembled inside trenches excavated below ground. The manholes are installed before the trench is excavated. Then the duct banks are assembled in situ in the trench, and the entire length of the assembled duct bank generally is encased totally in a concrete envelope before backfilling the trench with excavated soil, in order to restore the site to the original grade. Occasionally after the duct banks are assembled, the trench is backfilled with the excavated soil, instead of encasing the duct bank in concrete.
Workers must assemble the duct banks working inside the trench, which could be as deep as 13 to 16 feet below grade and deeper. Therefore, production and workman's safety are at risk, which means the construction costs are very high. OSHA regulations (29 CFR 1926.650, Appendix B to Subpart P, "Sloping and Benching") mandate the trench to be excavated with safety slopes of 1 to 1, varying to 2 to 1 and flatter, depending on the total depth of the trench and the soil characteristics, thereby to prevent the trench walls from collapsing on top of the workers.
In digging the trench with safety slopes, the digging operator has very little choice but to set the excavated soils on both sides of the trench.
Since the duct bank is assembled inside the trench, the workers assembling the duct bank inside the trench very often must stand and walk on the duct bank itself as it is being assembled. The workers in the trench also require the assistance of workers above the ground. The above-ground workers must carry 10 to 20 feet long pieces of conduit (duct) and other components required for the assembling of the duct bank while walking over the piles of excavated soil created when the safety slopes were excavated. This makes for low productivity and the requirement always to monitor safety conditions for the workers above ground.
Conduits typically are assembled in layers by using plastic spacers. Each plastic spacer rests on two bricks on the bottom of the trench.
If it rains, the excavated soil at the top of the trench runs back down into the trench as mud, thereby partially covering the partially assembled work inside the underground trench. In such a case, contractual terms and conditions obligate the contractor to disassemble the partially completed work, clean the trench, and start over again.
Installations of duct banks by conventional methods take several days for the completion of a distance of 500 feet, e.g., the distance typically between two precast concrete manholes. The trench must be excavated first. Next, the safety slopes are excavated on each side of the trench. Then, trench shoring or trench boxing is installed. The duct bank then is built in situ inside the trench. Depending on the soil characteristics and the depth of the trench, the trench in the traditional method is required to have trench shoring made of overlapping layers of sheets of plywood, with bracing across the trench and against 12 inches wide planks which in turn are set against the plywood sheets. Additional bracing also is required to be set against the sloping walls. Occasionally, when the trench is dug with straight walls, instead of trench shoring, trench boxing is utilized, which is made of steel plates kept apart by hydraulically or manually expanded bracing structures positioned across the trench and against the steel plates. The bracing structures are built in 20-feet long steel sections which must be moved by crane, along the trench, as the duct bank assembly progresses.
The trench shoring must be removed prior to pouring the concrete. Then forms must be built for pouring the concrete. The concrete is poured in such a manner as to encase the duct bank, by pouring the concrete in between the individual ducts as well as 3 inches all around the conduit's bottom, sides, and top, thereby encasing the entire duct bank in a concrete block. The forms must be removed prior to backfilling the trench.
Before pouring the concrete, conduits from each duct bank length built inside the trench must be tied into the manhole at each end of each length of duct bank in the trench. Manholes are made of precast concrete. The manhole wall thickness, at the tie-in points, varies depending on the manhole location and the potential axial loading, i.e., the weight which the manhole structure is designed to support. By the conventional method, the manhole walls are broken by sledge-hammering, which produces a large, very irregular opening on the manhole wall. A piece of plywood board is fixed temporarily against the inside of the precast manhole wall to cover the opening created by sledge-hammering. This plywood board is attached temporarily to the manhole inside wall by utilizing lead anchors on two-feet centers. Plywood board is removed after pouring the concrete. Next, one standard bell-end for each conduit on the duct bank is attached to the plywood board to coincide with the elevation and lateral position of the conduit to be tied into each standard bell-end. Each bell-end is tied-down against the board by means of tie-wires through holes drilled through the plywood board, thereby to prevent the bell-ends from moving too much when concrete is poured to encase the entire duct bank.
Straight conduit couplings, without a dividing rib on the inside, are inserted over the ends of each conduit from the duct bank. Couplings are inserted over the ends leaving a few inches exposed to apply an special adhesive later.
One end of an additional standard length of conduit is coated with a special adhesive and inserted into the bell-ends starting from the bottom layer of bell-ends. This insertion operation is performed from inside of the trench. The special adhesive sets in a matter of seconds. After each individual standard length of conduit is firmly glued to its respective bell-end, the conduit is saw-cut so as to have its other end approximately abutting to the end from the corresponding conduit from the duct bank.
Now the end from the corresponding conduit from the duct bank and the end from the conduit are coated with the special adhesive, and the coupling is tapped on one end, thereby to slide the coupling easily and quickly over both ends. This coupling completes the tie-in to the manhole for that one conduit from the duct bank. The overall operation is repeated for each conduit from the duct bank until all conduits are tied into the manhole.
After the above-described tie-ins are made, concrete is poured to encase the duct bank and to fill openings created by jackhammering on the manhole walls. Pouring starts from the center of the duct bank length, and pouring continues toward each manhole. After the concrete sets, the plywood boards are removed. Many irregularities remain on the surface of the inside wall of the manholes, and these irregularities must be patched.
Because the whole process typically takes several days for the completion of the installation by the conventional method, the trench must be barricaded and fully illuminated at night to prevent people from falling into it, until the installation is complete, i.e., until the duct bank has been fully assembled, the concrete has been poured to encase it, and the trench has been backfilled to the original grade.
In this manner, the conventional art method involves installing the duct banks inside trenches with safety slopes with or without trench shoring. Depending on the depth of the trench and the type of soils, the conventional methods also involve installing the duct banks inside trenches with straight walls by using steel trench boxing with hydraulically or manually expandable bracing across the trench. Whether the installation by conventional methods takes place inside trenches with sloped walls or inside trenches with straight walls with steel trench boxing, it is always done with a combination of workers performing the actual assemblage of the duct bank inside the trench, while other workers, stationed above ground, assist by handing down the component parts for the assembly of the duct bank to the workers inside the trench, stationed below ground, and while the above-ground workers walking over piles of excavated soils carry the components and further including performing sledge-hammering the walls of the manholes to complete the tie-ins, as described above.
There are many costly problems and drawbacks associated with the utilization of the conventional method. One problem is that the workers must assemble the duct bank while working inside a trench. The workers very often must stand and walk on the duct bank itself as it is assembled. Above-ground workers are needed to assist the workers inside the trench.
Another drawback is apparent in the requirement that any trench deeper than 4 feet, as mandated by OSHA regulations, must be excavated with safety slopes or otherwise be protected by installing an expensive steel trench boxing system.
A further drawback is the requirement to shore up the trench walls and to install forms for pouring the encasing concrete envelope and to remove both the shoring and the forms.
Conventional methods further require breaking the walls on each manhole by sledge-hammering or by other methods in order to create an opening for making the tie-ins at each end of the duct bank.
Conventional methods require many days to complete an installation. Another drawback with the lengthy time required in the conventional methods is weather dependency, especially in respect to rain conditions.
A further drawback is the requirement to barricade the trench and to illuminate it fully at night because it takes several days to assemble a standard five hundred (500) feet length of duct bank inside the trench.
Therefore, novel apparatus and method are needed to provide for the installation of underground duct banks for electrical cables or fiber optics cables or the like and to eliminate the problems and drawbacks of the conventional methods.
It is an object of the present invention to provide for the installation of underground duct banks by means and method which substantially reduce the assemblage of the duct bank inside the trench and which substantially reduce the amount of time needed for workers stationed inside the trench. A further object of the present invention is to eliminate the need for workers to walk on the underground duct bank as it is being assembled.
A further object of the present invention is to improve worker safety in the installation of underground duct banks.
A further object of the present invention is to provide for the installation of underground duct banks by means and method which eliminate the need to excavate the trench with slopes.
Another object of the present invention is to provide for the installation of underground duct banks by means and method which eliminate the need for trench shoring and trench boxing and which also eliminate the need for construction forms.
Another object of the present invention is to provide for the installation of underground duct banks by means and method which eliminate the need for above-ground workers to carry duct bank component parts over piles of excavated soil.
A further object of the present invention is to provide means and method for the installation of underground duct banks and eliminate the requirement to break manhole walls for making tie-ins at each end of the duct bank.
It is a still further object of the present invention to provide for the installation of underground duct banks and substantially reduce the number of days required to complete the installation process.
Yet another object of the present invention is to provide for the installation of underground duct banks and substantially reduce weather dependency.
Also an object of the present invention is to provide means and method for the installation of underground duct banks which eliminate the need to barricade the trench and also which eliminate the need to illuminate the trench at night.
These and other objects of the present invention will become apparent to those skilled in the art from a careful review of the detailed description which follows.